1. Field of the Invention
The present invention relates to methods of filtering. Specifically, the present invention relates to methods of filtering signals with a class-D amplifier.
2. Background Art
Conventional class-D amplifiers, or switching amplifiers, are important for applications that require high efficiency, broad bandwidth, and low signal distortion. Linear amplifiers such as class-A and class-AB amplifiers yield low signal distortion and broadband response, but are limited in maximum theoretical efficiency to 25% and 78.6%, respectively. In practice, class-AB amplifiers operate closer to 30% efficiency when driven with voice or music.
Pulsed linear amplifiers, such as a class-C amplifier, can yield efficiencies as high as 90%. But class-C amplifiers distort the signal in all but narrow band applications. Class-C amplifiers work well in conjunction with tuned circuits, as in radio frequency amplifies.
None of the aforementioned amplifiers is as efficient as a class-D amplifier, with practical efficiencies of 95% or higher. Further, class-D amplifiers replicate the input signal over a broad band, a limiting factor for class-C amplifiers.
One of the drawbacks to conventional class-D amplifiers is that a 50% duty cycle square wave is driven to the output when no input signal is present. With no filter, the square wave appears across the load as a DC voltage, resulting in a finite load current, increasing power consumption. In many cases, providing the filter to remove the DC component is prohibited by space and packaging limitations.
Filterless class-D amplifiers address DC power consumption concerns by providing differential pulse width modulation (PWM) of the input signal. As a result, the output signal is driven to zero when no input signal is present, substantially reducing power consumption. Filterless class-D amplifiers accomplish this by providing two pulses per period of the comparative signal. Pulses are generated in accordance with a state machine, triggered by clock and reference signal inputs.
In contrast to conventional class-D amplifiers, filterless class-D amplifiers produce very narrow pulses, nearly eliminating DC power consumption. However, filterless class-D amplifiers generate high frequency dynamic switching noise, a direct result of driving narrow pulses at the output. Some, but not all of the high frequency dynamic switching noise above 20 kHz is rejected by voice coils due to their inductance, standard in most speakers. What is needed is a filterless class-D amplifier that eliminates DC power consumption and significantly reduces high frequency noise at the output.